One hypothesis proposes that mitochondria evolved from endosymbiotic prokaryotes that were able to use oxygen to generate energy-rich ATP. Inside primitive eukaryotic cells, these energy-generating prokaryotes evolved into mitochondria that now power the cells of all multicellular organisms. Mitochondria enabled cells to metabolize oxygen. Without this ability, cells would have been killed by the free oxygen in the atmosphere.

Another hypothesis proposes that chloroplasts evolved from endosymbiotic prokaryotes that had the ability to photosynthesize. Over time, these photosynthetic prokaryotes evolved within eukaryotic cells into the chloroplasts of plants and algae.

Modern Evidence During the 1960s, Lynn Margulis of Boston University gathered evidence that supported the endosymbiotic theory. Margulis noted first that mitochondria and chloroplasts contain DNA similar to bacterial DNA. Second, she noted that mitochondria and chloroplasts have ribosomes whose size and structure closely resemble those of bacteria. Third, she found that mitochondria and chloroplasts, like bacteria, reproduce by binary fission when cells containing them divide by mitosis. Mitochondria and chloroplasts, then, share many features of free-living bacteria. These similarities provide strong evidence of a common ancestry between free-living bacteria and the organelles of living eukaryotic cells.

In Your Notebook Describe two hypotheses relating to the endosymbiotic theory.

FIGURE 19–18 The Endosymbiotic Theory The endosymbiotic theory proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms. Ancient prokaryotes may have entered primitive eukaryotic cells, remained there, and evolved into organelles. Infer Is it likely that nonphotosynthetic prokaryotes could have evolved into chloroplasts? Explain your answer.

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